Transceivers are transmitter (or driver) receiver units that connect a host or other intelligent controller to a set of peripheral devices. Such transceivers are usually composed of bidirectional driver receiver pairs or channels, with each channel providing an interface between differential signal levels transmitted via input lines, such as may be found in twisted pair cables, and internal logic signal levels within the machine or computer to which the transceiver is coupled. The bidirectional driver outputs are connected across the input lines in the interface to the receiver inputs. The receiver outputs are connected to the driver inputs.
Such transceivers require that any open input lines be detected because the noise immunity of such differential systems is dependent on the presence of both signals in the differential pair. In such systems, the absence of one of the differential signals due to an open in the input lines often results in random, difficult-to-reproduce transmission errors. It is therefore, imperative that such opens be immediately detectable.